Adhering structural elements using various glues is a widely used method of attachment, in particular of structural elements made from two different types of materials, for example, a composite material and a metal. One of the major problems of such an attachment is the separation or partial separation of the two structural elements due to weakening of the adhesion strength between joint surfaces of the structural elements. This separation or weakening is difficult to detect, in particular during the service life of the structural elements. For example, when the two structural elements are included in a vehicle (e.g., an aircraft, maritime vessel or a car) it is almost impossible to detect the weakening when the vehicle is on the move (e.g., drive, dive, sail or fly).
Currently known methods use load pins (LPs) for measuring loads (e.g., dynamic loads) in structures. The LPs are mounted to the structure such that the LPs are carrying loads derive from the loads acting on the joint structure. Accordingly, such an LP measures, in high accuracy, dynamic loads acting on the structure. For example, an LP located in a wing of an aircraft will measure dynamic loads acting on the wing during flight.
When coming to detect separation or weakening of the adhesion strength between two elements (e.g., two elements in a wing) a different approach may be taken. Measuring the actual loads acting on the entire joint structure (e.g., wing) cannot indicate if a separation or weakening of the adhesion has occur since it does not measure the adhesion itself. While the joint structure may be heavily loaded the adhesion may still hold the two elements without a failure.
There are several known methods of detecting separation or weakening using direct measurements of the adhesion strength by placing sensors inside an adhesion layer between the two structural elements. A strain gauge (e.g., piezoelectric device) may be embedded in the adhesion layer, directly sensing changes in loads acting in the adhesion layer. This method is limited to the area of which the sensor is inserted and may weaken the adhesion strength by inserting an alien body into the adhesion layer. Additional method is the use of acoustic sensors placed at various areas in the structural elements. Due to the size of the acoustic sensors and the accompanied equipment, this method is very limited and cannot be applied in structures having limited spacing and accessibility that are unapproachable and located in closed assemblies. Additionally, acoustic methods can be used only when the assembly (the vehicle) is stationary.
Accordingly, there is a need for a simple system for measuring separation or weakening of the adhesion between two or more elements in a joint structure. Such a system according to some embodiments of the present invention may use LPs to indirectly detect a separation or weakening of the adhesion between two or more elements in a joint structure. An LP according to some embodiments of the present invention may be inserted to the joint structure such that the LPs itself does not measure the dynamic loads acting on the joint structure and initially (when the adhesion strength is suffice for holding the joint structure) no load at all is acting on the LP even if the joint structure is dynamically loaded.